Continuous circulating crystallization process and apparatus



May 6, 1958 R. M. GREEN x-:TAL

CONTINUOUS CIRCULATING CRYSTALLIZATION PROCESS AND APPARATUS Filed NOV. 19, 1954 A 7' TORNEVS United tates Patent CONTINUOUS CIRCULATING CRYSTALLIZATION PROCESS AND APPARATUS Richard M. Green and Joseph W. Clark, Borger, Tex.,

assignors to Phillips Petroleum Company, a corporation of Delaware Application November 19, 1954, Serial No. 469,976

14 Claims. (Cl. 260-674) This invention relates to a method of forming crystals from a solution containing crystallizable material. in a further aspect this invention relates to apparatus for carrying out this process. In a further aspect this invention relates to a crystallizer for forming large crystals comprising a tube mounted within a crystallization zone, scraper blades attached to this tube adapted to scrape the walls upon which crystals deposit, and impeller means for circulating a liquid slurry past the heat exchange area of said crystallization zone. In a further aspect this invention relates to a process for the purification of materials by crystallization.

The use of crystal purification columns of the type disclosed in Schmidt Re. 23,810, Weedman application Serial No. 166,992, now Patent No. 2,747,001, tiled June 9, 1950, and McKay application Serial No. 375,850, filed August 24, 1953, has become widespread. Such columns provide a method for producing very pure compounds, frequently above 99 percent purity. Many dirferent systems have been employed to prepare the crystal feed to such purification columns. For etiicient operation it is necessary that the crystals fed to the column be of relatively large size. One form of apparatus which has been used to prepare crystals is a scraped surface chiller. A difliculty resulting from the use of such a chiller is that the crystals are of very small size. lt has been suggested that the effluent from such a chiller be subjected to a holding operation in order to permit growth of crystal size. However, this involves `additional equipment and the resultant additional ex- .uuaided process of melting and recrystallizing is slow.

The following are objects of this invention.

An object 4of this invention is to provide an improved process forthe production of crystals. A further object of this invention is to provide apparatus for producing such crystals. A further object of this invention is to provide a method for forming crystals comprising maintaining a comparatively large volume of crystal slurry in motion, adding feed to this volume of circulating slurry thereby increasing the size of the crystals present preferentially to` forming new and small crystals. by growth of crystals already present, and maintaining good heat transfer to the circulating slurry by preventing a layer of crystals from forming upon heat exchange surface. A further object of this invention is to provide a process for the purication of materials by crystal- Vlization.

Other objects and advantages of this invention will be vapparent to one skilled in the art upon reading this specification. p

Accompanying and forming a part of this speciiica- "tion is a drawing illustrating schematically the apparatus lof this invention and showing, partly in section, the

:improved chiller which we have invented.

We have discovered an improved pro-cess of and apric paratus for forming crystals. This comprises modifying a scraped surface chiller so that a stream of the crystal slurry passes by the heat exchange surface at a rapid rate of speed. Feed material introduced to this stream is rapidly cooled and the crystallizable components thereof tend to deposit on existing crystal nuclei rather than forming new nuclei. Furthermore, circulation at a high rate reduces the temperature gradient in the apparatus.

The invention can best be understood from the drawing accompanying this specification wherein the principal components for the separation process comprise the chiller lil, a rotary filter 11 and a purication column 12.

The crystallizer 10 comprises an outer chamber 13 through which heat exchange material is passed, .conduits 14 and 16 providing for introduction of and removal of the heat exchange material. In the usual operation this heat exchange material comprises a refrigerant although heat is added n for materials with negative solubility coefcients, i. e. compounds which become less soluble as the temperature of the solvent increases. Inside of the chiller there is provided a tube 17, this tube being rotatably mounted within the chiller and provided with at least one scraper 18 adapted to scrape the wall 15 of the heat exchanger. The area of wall 15 contacted by the scraper blades corresponds substantially to the area contacted by the heat exchanger. Positioned within tube 17 we provide, in the preferred modification, impellers 19 and 2l, these being mounted upon shaft 22. ln the drawing, we have illustrated a method by which both tube 17 and shaft 22 can be driven from a single source of power 23. As will be pointed out hereafter it is desirable that the impeller operate at a very high speed while the tube rotates slowly. Although separate drive means could be used for these actuated elements, a single power source can be used. lf a large gear 24 is provided on the motor shaft 26 for driving a small gear 27 on impeller shaft 22, and if a small gear 28 is provided on motor shaft 26 for driving a large gear 29 on tube 17, it will be seen that the tube turns slowly while the impeller turns rapidly, both in the same direction. The upper end of tube 17 is provided with a series of openings 31 of a size suicient to permit flow of the crystal slurry therethrough. One end of scraper blades 18 is attached to an annular ring 32 and the other end of these blades is attached to an open bearing plate 33, this bearing plate riding upon roller bearings 34. A feed inlet conduit 36 is provided and a slurry outlet conduit 37 is also provided. A cleanout conduit 38, containing Valve 39, extends from the lower end portion of the chiller. Conduit 38 can be used as an alternate slurry outlet.

The remainder of the apparatus, shown to complete the disclosure of the entire process, comprises the rotary lter 11 and the crystal purification column 12. Conduit 37 extends into the pan 41 in which the rotating iilter member 42 rotates. Conduit 43 conveys material from the rotary ilter to the purication column 12 and conduit 44 is provided to remove mother liquor obtained in the filtering operation. Conduit 43 communicates with column feed conduit d6, this feeding the solid material to the purication column l12 wherein it is moved downwardly by piston 47. Means for supplying heat to the lower portion of column 12 is provided, such as a heat coil 4S. Product removal conduit 49 is shown extending from the lower portion of column 12, this conduit being provided with valve 51. Recycle conduits 52 and 53 are provided and either or both of these conduits can be utilized in a specic adaptation of this apparatus. ln operation, heating coil 48 melts the solid material and a` portion of this is withdrawn "from feed 4conduit 36. course, communicate with the 'then lower portion of the crystallizer. y"means caribe substituted forthe propeller blades 19 and V21. Turbine blades can be used as well as `other kinds -of pumps. -tical position, as shown in the drawing, an air jet can be usedto circulate the slurry, such an air jet introducing 15 to A20 minutes.

'as the product, the remainder being forced upwardly displacing koccluded impurities from the solid. Conduit 52 is utilized when a porous piston 47 is used and, if a solid piston is used, the liquid can be removed through conduit 53. Conduit 53 communicates with liquid collection means 54, a lilter being provided in the wall of the purification column for removal of liquid therefrom. In this drawing conduits 52 and 53 join to form conduit 56. Since the liquid removed from this column is usually relatively rich in the material which is being purified, 'conduit 56 is shown communicating with feed `conduit 36.

In the drawing we have shown a vertical crystallizer `10 but it will be understood that this piece of apparatus 'could beV operated in a horizontal or, lin fact, in any other position. In horizontal operation it ispreferable to have theslurry'removal condut`37 at the opposite end Drain conduit 38 should, of

Furthermore, equivalent slurry impeller When the -crystallizer is operated inthe verair into the bottom `of the crystallizer, the crystallizer being provided with a vent at the top thereof.

In the-operation of this crystallizer the slurry contain- .ing zone is lled with feed material and refrigerant is circulated throughthe zone 13. When the temperature in the crystallizer becomes low enough crystals start to form. When these reach a suitable size, the apparatus .can Vbe operated continuously to provide a slurry feed for the rotary lter 11. This is done by removing a portion `of the slurry through conduit 37 or conduit 38 and introducing an equivalent amount of feed liquid through conduit 36. The crystallizer should be of suicient size to -give a residence time of at least 30 minutes, a minimum Vtime of 45 minutes frequently being more suitable.

The maximumy residence time is of lesser importance but is usually in the range of 2 to 4 hours.

The crystallizer is constructed so as to provide an annular space between tube 17 and the wallv 1S of the crystallizer of smaller size than that of the tube. We

.prefer to have the cross section area of the annulus 1/s to 1A ofthe -crosssection area kof the tube. This provides for the relatively high speed necessary alongthe walls offthe crystallizer. 'This speed is necessary since it tends to preventdeposition of' solid material onthe wall 15.01?

change surface and this minimizes crystallization and nncleation in the annulus.

When the crystallizer is operated in -a vertical position as shown in the drawing there is an additional factor which must be considered in determining the speed of lliquid ow through the apparatus. It will be evident that,

for eicient operation, the slurry flow must be at a speed in excess of the settling rate of crystals in the slurry. Otherwise solid material will collect in the lower portion of the apparatus.

The tube-containing the scraper blades is rotated slowly and,l generally, at such a speed to clean therefrigerated tube Wall once every to 60 minutes, preferably every Since the clean wall surface .offers an area for theformation of small crystal nuclehthe wall should not be cleaned oftener than necessary to prevent excessive crystal buildup. The maximum time between each operation of cleaning the wall is ak function of the 'amount of crystal buildup thereon because .alayer of crystals on this wall reduces the heat 'transfer there- 'through to` a great extent.

ing large crystals of paraxylene from a feed material which is a -conventional xylene mixture containing 16 to 18 percent para-xylene; the remainder being meta-xylene, ortho-xylcne, and ethyl benzene. The crystallizer, having a capacity of 6 gallons, is first filled and cooled. When the temperature of the slurry reaches l00 E., feed is added continuously at a rate of 8 gallons per hour, providing a residencetime of 45 minutes. Large crystals are formed and, following 40 minutesofoperation, the temperature in the crystallizer isat- F., the temperature of the refrigerant `being 112 F. The heat transfer coefficient (B. t. -u./hr.'/sq. ftl)'is28,'with substantially no deposit on the refrigerant wall. With scraping every 40 minutes, this heat transfer coeliicent is maintained at substantially 'a constant ligure and large crystals can .be obtained. If thescraper is stopped -and the otherconditions maintained ,the same, theheat transfer coe'ircient drops rapidly'and,.after 6 hours isreduced to 8 B. t. u./hr./sq. ft. This lowA heat transfer coeiiicient is the result of the building up of a layer of crystals on the refrigerant wall approximately 0.1 inch in thickness.

From these data it is apparent that agitation alone does not provide a method for continuous production of the crystal slurry. it would .be necessary to shut down the system `and melt off the accumulated wall layer in order that an effective heat transfer coefficient bemaintained. However, with thescrapingthe apparatus will function in a continuous manner. Usingthis apparatus it is possible to obtain crystals as large as 0.2 :r0.6 mm. with some 0.6 x 016 mm. clusters being obtained. For best results in the puricationcolumn, crystals should have an' average maximum dimension of at least 0.2 mm. Large crystals have theadvantage of providing less surface per unit weight on-which mother-liquor is occluded.

The slurry'from the 'crystallizer is` introduced into pan 41 of a rotary filter, such as an Oliver lter. The filter cake formed on this lter is removed and introduced by conduits 43 and 46 into crystal purification column 12. By the operation -of piston 47 -a compactmass of crystals is moved downwardly in the column. Heat supplied by heater 48 melts these crystals anda portion islwithdrawn as product through conduit 49. A portion ofthe melt is forced upwardly in the column and this portion dis- Vplaces occluded impurities from the compact Vmass and the resulting liquid is removed from the upper portion of the column, is mixed with the feed, and introduced again into the crystallizer. For a more complete description of the operation of the crystal purication-column, attention is directed to the prior artset forthabove.

This crystallizer represents a considerable advantage `over the use of the conventional scraped surface chiller operation in which the feedy is passed through a cooling zone to form crystals in a one-pass operation. In such a method, a large number of nuclei always form at the` point where crystallization begins.

The present invention overcomes this inherent disadvantage by cooling the feed to the crystallization temperature in the presence of existing crystals. Therefore, when the crystallization temperature is reached, nuclei are already present on which the material can deposit.

The process and apparatus of our invention are applicabletoa vast number of separations. The invention is applicable -to, in general, mixtures of compounds which are ditlicult to separate by conventional means such Ias fractional distillation. The invention is applicable to mixtures which form eutectics. Fromia consideration of rthe phase diagram of a binary system which forms a eutectic it is obvious that either component (dependingiupon the composition of the specific mixture) can beseparated by freezing until the compositionof the mother liquor reaches the approximate eutectic point. Itis also apparent that effective separationofthecomponentscan be made from lsystems where the concentration of one 'component is relatively low. 1n order to illustrate a few of the systems to which the invention is applicable, the following compounds are grouped with respect to their boiling points:

B. P., F. P., C. C.

Group A:

Benzene 80 5. 5 n-Hexane 69 -94 n-Heptane 9S. 52 90. 5 Carbon tetrachloride. 77 22. s Acrylonitrile 79 82 Ethyl alcohol 78. 5 117. 3 2,2-Dlmethylpentane 79 -125 3,3-Dimethylpentane 86 Methyl ethyl ketone 79. 6 -86. 4 Methyl propionate 79. 9 -Sr 5 Met y1aerylate 80. 5 1,3-Cyclohexadiene S0. 5 -98 2,4-Dimethylpentane 80. 8 -123. 4 2,2,3-Trimethylbutane. 80. 9 25 yclohexane 81. 4 6. 5 Acetonitrile 82 42 Cyclohexene. 83 103. 7 2-Methy1hexane 90 -119 S-Methylhexane 89. 4 119. 4 Group B:

Methyl eyclohexane. 100. 3 -126. 3 Cyclohexane S1. 4 6. 5 n-Heptane 98. 52 90. 5 2,2,4-Trmethylpentane (isooctane) 99.3 107. 4 Nitromethane 101 p-Doxane 101. 5 11. 7 2-Pentanone 101. 7 77. 8 2-Methyl-2-butanol 101.8 `11. 9 2,3-Dimethylpentane.- 89. 4 B-Ethylpentane 93. 3 94. 5 Group C:

Toluene 8 Methylcyclohexane.- 3 2,2,3,3Tetramethyl butano- 8 2,5Dimethylhexane 2,4-Dimethylhexane 2,3-Dimethylhexane-- 3,4-Dimethylhexa ne 3-Ethyl-2-methylpentane.- 3Ethyl3methylpentane Group D:

77 22. 8 Chloroform 61 -63. 5 C 46.3 108.6 Acetone 56. 5 -95 Group F:

ortho-Xylene 144 27. 1 meta-Xylene 138. 8 -47. 4 para-Xylene.-- 138. 5 13. 2 Group G:

ortho-Cymene 175. 73. 5 meta-Cymene 175. 7 -25 para-Cymene-.. 176.0 -73.

Systems consisting of any combination of two or more of the components within any one of the groups may be separated by the process of the invention as well as systems made up of components selected from different groups; for example, benzene may be separated from a benzene-n-hexane or n-heptane system in which the benzene is present in an amount greater than the eutectic concentration. Benzene may also be separated from a mixture of toluene and/or aniline. Multi-component systems which may be effectively separated so as to recover one or more of the components in substantially pure form include 2,2-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane; methyl cyclohexane, 2,2,4- trimethylpentane; and carbon tetrachloride, chloroform, acetone. The invention is also applicable to the separation of individual components from a system of cymenes.

It should be understood that many so-called binary systems also include small percentages of one or more other compounds as impurities which may be practically disregarded as far as operation of the process is concerned since they do not freeze out with the crystals but are retained in the mother liquor.

Our invention can also be used for the separation and purification of food products, such as vegetable juices, fruit juices, such as orange juice, and beverages, such as wine, beer, coffee, tea, etc.

lt will be apparent that various modifications in addi- 'tion to those set forth above can be made in this process and apparatus by those skilled in the art while still carrying out the essential features of our invention.

We claim:

l. Apparatus for continuously forming crystals in a slurry, comprising a closed chamber; a heat exchange jacket surrounding said chamber, a tube rotatably mounted within said chamber constructed and adapted to permit liquid flow between the space inside of said tube and the space between said tube and said chamber at each end of said tube; means to `drive said tube; at least one scraper blade attached to said tube adapted to scrape the inner wall of said chamber; impeller means adapted to circulate material around said vtube in a longitudinal direction; means to drive said impeller; a feed material inlet conduit communicating with the space between said tube and said chamber; and an youtlet conduit communicating with the space between said tube and said chamber.

2. The apparatus of claim l associated with a crystal purification system comprising means to separate crystals from said slurry and a crystal purification column.

3. Apparatus for continuously forming crystals of large size in a slurry, comprising a closed chamber; a heat exchange jacket surrounding said chamber; a tube rotatably mounted within said chamber constructed and adapted to permit liquid flow between the space inside of said tube and the space between said tube and said chamber at each end of said tube; means to drive said tube; at least one scraper blade attached to said tube adapted to scrape the inner Wall of said chamber; at least one impeller rotatably mounted within said tube; means to drive said impeller; a feed material inlet conduit communicating with the space between said tube and said chamber; and an outlet conduit communicating with the space between said tube and said chamber.

4. Apparatus for continuously forming crystals of large size in a slurry, comprising a substantially vertically disposed closed chamber; a heat exchange jacket surrounding said chamber; a tube rotatably mounted within said chamber constructed and adapted to permit tlow between the space inside of said tube and said chamber at each end of said tube; at least one scraper blade attached to said tube adapted to scrape the inner wall of said chamber; at least one impeller rotatably mounted within said tube; means to drive said tube and said impeller; a feed material inlet conduit extending into the upper end portion of said chamber; and an outlet conduit extending from the upper end portion of said chamber.

5. Apparatus for continuously forming crystals of large size in a slurry, comprising a substantially vertically closed chamber; a heat exchange jacket surrounding said chamber; a tube rotatably mounted within said chamber constructed and adapted to permit liquid ow between the space inside of said tube and the space between said tube and said chamber at each end of said tube; a plurality of scraper blades attached to said tube adapted to scrape the inner wall of said chamber; a plurality of impellers rotatably mounted within said tube; means to -drive said tube and said impellers; a feed inlet communieating with the upper end portion of said chamber; a rst outlet conduit communicating with the upper end of said chamber; and a second outlet conduit communicating with the lower end portion of said chamber.

6. Apparatus for continuously forming crystals in a slurry, comprising a closed chamber, a heat exchange jacket surrounding said chamber; a tube rotatably mounted within said chamber constructed and adapted to permit liquid ow between the space inside of said tube und the space between said tube and the said chamber at each end of the tube, the cross section area of the space between said tube and the said chamber being smaller than the cross section area of said tube, means to drive said tube; at least one scraper blade attached to said tube adapted to scrape the inner wall of said chamber; impeller means adapted to circulate material around `said ansa-n35 7 tubenlzaglongitudinal direction-means to drive saidimpe11er;.;a material inletconduitcommunicating lwith .the spacezbetweensaid .tubeand said. chamber; :andanoutlet conduit communicating with the space betweenrsaidr-tuhe k-ancl said chamber.

7.,A continuous processfor Aforming.acrystal .slurry .comprising introducing liquid feed to a circulatingslurry A.in a .crystal formingzonecontaining a heatexchange sur- .'face, saidv slurryrcirculating along a path .withi-nsaid zone but isolatedafrom said heat exchangesurfaceand ret-urn- .ing adjacent said heat exchange surface, theispeed of said .slurry rbeingsucient .to minimize deposition of crystals son said heatexchange surface; periodically removing crys- A tals deposited upon said heat .,exchangesurface; .adding Afeed to said circulating slurry;..and withdrawing .a portion ofsaid crystal. slurry. for, further .desi red treatment.

8. .A .continuous process for forminga crystal slurry from a, solution comprising. circulatingina .crystal forming zone a crystal .slurry at .a -speed `in excess of the settling rate of .solids in,saidslurrylongitudinallypast a'heat exchange surface, the return flow of :saidjiguid being along a path Within said zone.hutrisolatedffromsaid ,ow past said heat exchange surface, the speed .of said slurry being suicient to minimize deposition of crystals on said heat'exchange surface; periodically removing.

crystals deposited upon=saidheat exchange surface; add- ,ing .'feed to said circulating slurry; and withdrawing a y.portionof said crystal slurry for lfurther desired treatment.

"9. A continuousprocess for `forming a crystal "slurry from a'solution, comprising circulatingin'a crystal'forming zone a crystal. slurry past .a'heat exchange surface. atta `linear velocity of 0.5 to 20 feet `per second, the return flow of said liquid being along a path within said zone but isolated from said ow past said'heatexchange surface, said v speed of said slurry 'being Asufficient to minimize .deposition .of ycrystals on said .heat exchange surface; removing crystals .deposited on saidheat exchange surfaces: at .intervals 4from .5 .to r60 minutes; Yadding s feed t to said` circulating slurryl and allowing thelcrystallizablecom- ,ture comprises paraand meta-xylenes.

E,pcmentofthe feedz-to deposit on-.crystal nuclei-already iforme'd,..and withdrawing .a .portion of .said crystal slurry ..for.`further;.'desired ,treatment, .thefeed yand withdrawal fmoving crystals deposited on said heat vexchange surfaces at intervals from l5 to 20 minutes; adding yfee'd to'said circulating slurry and allowing the crystallizallecomponent of the feed tofdeposit-on crystal ynuc'leivalready {ormed,rand withdrawing a portion of said crystal slurry for further desired treatment, the fee'd'and withdrawal -rates being such that an average residence -tirneff 45 minutes .to 2 hours is maintained.

ill. The -:process ofclaim l0 in whichsaid liquidxmixl2. Thefprocess of claim l1 in which para.xylcne is separated/.as a pure-product.

i3. Theprocess .of claimlOfin which..said..liquidcmix ture comprises `a mixture of 'paran orthof, and :metaxylene.

14. Thergprocess of claim Y1-3 vinwhichpara`--xylene is separated as the pure product.

References 'Cited in thele o'fathis patent UNITED STATES 'PATENTS 2,552,625 ,Weuzelberger May 31"5, :1951

2,617,273 Findlay -Nov. l-l, :1952

2,672,487 Tegge et al Mar.`;l6,il\9-54 '2,683,178 Findlay VInh/26, 1.1954

'FOREIGN PATENTS 4605834 'Great Britain Feb. r4, 1937 

1. APPARATUS FOR CONTINUOUSLY FORMING CRYSTALS IN A SLURRY, COMPRISING A CLOSED CHAMBER; A HEAT EXCHANGE JACKET SURROUNDING SAID CHAMBER, A TUBE ROTATABLY MOUNTED WITHIN SAID CHAMBER CONSTRUCTED AND ADAPTED TO PERMIT LIQYID FLOW BETWEEN THE SPACE INSIDE OF SAID TUBE AND THE SPACE BETWEEN SAID TUBE AND SAID CHAMBER AT EACH END OF SAID TUBE; MEANS TO DRIVE SAID TUBE; AT LEAST ONE SCRAPER BLADE ATTACHED TO SAID TUBE ADAPTED TO SCRAPE THE INNER WALL OF SAID CHAMBER; IMPELLER MEANS ADAPTED TO CIRCULATE MATERIAL AROUND SAID TUBE IN A LONGITUDINAL DIRECTION; MEANS TO DRIVE SAID IMPELLER; A FEED MATERIAL INLET CONDUIT COMMUNICATING WITH THE SPACE BETWEEN SAID TUBE AND SAID CHAMBER; AND AN OUTLET CONDUIT COMMUNICATING WITH THE SPACE BETWEEN SAID TUBE AND SAID CHAMBER. 